[FS#966] ar71xx: images are not named to their board_names

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - Alexander Couzens (lynxis) 

Attached to Project - LEDE Project
Summary - ar71xx: images are not named to their board_names
Task Type - Bug Report
Category - Base system
Status - New
Assigned To - 
Operating System - All
Severity - Low
Priority - Medium
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - Create an image using the imagebuilder (e.g. ar71xx/generic)

- make PROFILE=ubnt-loco-m-xw image
- the resulting image is than named lede-ar71xx-generic-ubnt-loco-m-xw-squashfs-factory.bin
- but looking into /tmp/sysinfo/board_name it's called "loco-m-xw"

original reporter: aparcar
Additional reference: https://github.com/aparcar/gsoc17-attended-sysupgrade/issues/26#issuecomment-321918497

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=966